The present invention relates to integrated circuits formed in semiconductor materials and in particular relates to methods for forming via openings in semiconductor substrates, Group III nitride epitaxial layers, and the resulting structures. More particularly, the invention relates to the use of such vias to form monolithic microwave integrated circuits (MMICs) in silicon carbide (SiC).
The present invention relates to the manufacture of via openings (“vias”) in integrated circuits (ICs), and in particular relates to a method of forming such vias in devices on a silicon carbide substrate in order to take advantage of silicon carbide's electronic, thermal, and mechanical properties in the manufacture and use of monolithic microwave integrated circuits.
MMICs
In its most basic sense, a monolithic microwave integrated circuit is an integrated circuit; i.e., a circuit formed of a plurality of devices; in which all of the circuit components are manufactured on top of a single semiconductor substrate, and which is designed to operate at microwave frequencies. As is generally the case with integrated circuits, the advantage of placing the device and circuit components on a single substrate is one of saving space. Smaller circuit size offers numerous advantages for electronic circuits and the end-use devices that incorporate such circuits. In general, the end-use devices can be smaller while offering a given set of functions, or more circuits and functions can be added to devices of particular sizes, or both advantages can be combined as desired. From an electronic standpoint, integrated circuits help reduce or eliminate problems such as parasitic capacitance loss that can arise when discrete devices are wire-bonded to one another to form circuits. These advantages can help integrated circuits operate at improved bandwidths as compared to circuits that are “wired” together from discrete components.
Wireless communications systems represent one area of recent and rapid growth in integrated circuits and related commercial technology. Such systems are exemplified, although not limited to, cellular radio communication systems. One estimate predicts that the number of wireless subscribers for such phones will continue to grow worldwide and will exceed 450 million users in the immediate future. The growth of such technologies is will require that devices are smaller, more powerful and easier to manufacture. These desired advantages apply to base, relay and switching stations as well as to end user devices such as the cellular phones themselves.
As recognized by those of ordinary skill in this art, many wireless devices, and in particular cellular phone systems, operate in the microwave frequencies of the electromagnetic spectrum. Although the term “microwave” is somewhat arbitrary, and the boundaries between various classifications or frequencies are likewise arbitrary, an exemplary choice for the microwave frequencies would include wavelengths of between about 3,000 and 300,000 microns (μ), which corresponds to frequencies of between about 1 and 100 gigahertz (GHz).
As further known by those of ordinary skill in this art, these particular frequencies are most conveniently produced or supported by certain semiconductor materials. For example, although discrete (i.e., individual) silicon (Si) based devices can operate at microwave frequencies, silicon-based integrated circuits suffer from lower electron mobility and are generally disfavored for frequencies above about 3-4 GHz. Silicon's inherent conductivity also limits the gain that can be delivered at high frequencies.
Accordingly, devices that operate successfully on a commercial basis in the microwave frequencies are preferably formed of other materials, of which gallium arsenide (GaAs) is presently a material of choice. Gallium arsenide offers certain advantages for microwave circuits and monolithic microwave integrated circuits, including a higher electron mobility than silicon and a greater insulating quality.
Because of the frequency requirements for microwave devices and microwave communications, silicon carbide is a favorable candidate material for such devices and circuits. Silicon carbide offers a number of advantages for all types of electronic devices, and offers particular advantages for microwave frequency devices and monolithic microwave integrated circuits. Silicon carbide has an extremely wide band gap (e.g., 2.996 electron volts (eV) for alpha SiC at 300K as compared to 1.12 eV for Si and 1.42 for GaAs), has a high electron mobility, is physically very hard, and has outstanding thermal stability, particularly as compared to other semiconductor materials. For example, silicon has a melting point of 1415° C. (GaAs is 1238° C.), while silicon carbide typically will not begin to disassociate in significant amounts until temperatures reach at least about 2000° C. As another factor, silicon carbide can be fashioned either as a semiconducting material or a semi-insulating material. Because insulating or semi-insulating substrates are often required for MIMICS, this is a particularly advantageous aspect of silicon carbide.
Advances in semiconductor electronics have increased the availability of wide-band gap materials, such as silicon carbide (SiC) and the Group III nitrides (e.g. GaN, AlGaN and InGaN). The potential for producing transistors operating at high frequencies, including the microwave band, has therefore become a commercial reality. Such higher frequency devices are extremely useful in a number of applications, some of the more familiar of which are power amplifiers, wireless transceivers such as cellular telephones, and similar devices. See generally, commonly assigned U.S. Pat. No. 6,507,046.
The wide bandgap characteristics of silicon carbide and the Group III nitrides enable device manufacturers to optimize the performance of semiconductor electronics at frequencies that traditional materials can not withstand. The high frequency capabilities of these wide bandgap materials present opportunities for development of high frequency, high power semiconductor electronic devices on a scale that will meet the needs of a growing industry.
Wide band gap epitaxial layers of significant interest include the Group III nitrides that are capable of withstanding operation at microwave frequencies. Wu and Zhang explain the operation of these wide bandgap epitaxial layers in international patent application WO 01/57929, assigned to Cree Lighting Company, a wholly owned subsidiary of the assignee herein. Of particular importance to Wu and Zhang are high electron mobility field effect transistors, known as HEMTs. HEMTs, as shown in WO 01/57929, comprise an upper epitaxial layer of semiconductor material on an insulating layer. Source, drain and gate contacts are fabricated on the upper epitaxial layer. The HEMT takes advantage of the physical phenomenon that occurs when two chosen materials of different band gaps are placed in contact with one another in an electronic device. The upper epitaxial layer in an HEMT typically has a wider bandgap than the insulating layer underneath it, and a two dimensional electron gas (2DEG) forms at the junction between the upper epitaxial layer and the insulating layer. The 2DEG formed at this junction has a high concentration of electrons which provide an increased device transconductance. The 2DEG serves as the channel of an HEMT. This channel is open and closed depending on the bias of the signal applied to the gate electrode. See WO 01/57929.
HEMTs are useful in applications that require high power output from a high frequency input signal. HEMT devices can generate large amounts of power because they have high breakdown fields, wide bandgaps (3.36 eV for GaN at room temperature), large conduction band offset, and high saturated electron drift velocity. The same size GaN amplifier can produce up to ten times the power of a GaAs amplifier operating at the same frequency. See WO 01/57929.
The 2DEG of a high electron mobility transistor is essentially an electron rich upper portion of the undoped, smaller bandgap material under the wider bandgap epitaxial layer. The 2DEG can contain a very high sheet electron concentration on the order of 1012 to 1013 carriers/cm2. See commonly assigned U.S. Pat. No. 6,316,793. Electrons from the wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility in this region. Id. A major portion of the electrons in the 2DEG is attributed to pseudomorphic strain in the AlGaN; see, e.g., P. M. Asbeck et al., Electronics Letters, Vol. 33, No. 14, pp. 1230-1231 (1997); and E. T. Yu et al., Applied Physics Letters, Vol. 71, No. 19, pp. 2794-2796 (1997).
High power semiconducting devices, such as the above described HEMT, operate in a microwave frequency range and are used for RF communication networks and radar applications. The devices offer the potential to greatly reduce the complexity and thus the cost of cellular phone base station transmitters. Other potential applications for high power microwave semiconductor devices include replacing the relatively costly tubes and transformers in conventional microwave ovens, increasing the lifetime of satellite transmitters, and improving the efficiency of personal communication system base station transmitters. See commonly assigned U.S. Pat. No. 6,316,793.
Accordingly, the need exists for continued improvement in high frequency, high power semiconductor based microwave devices. One significant improvement described in detail herein is the development of a means for fabricating HEMT devices as part of a monolithic microwave integrated circuit (MMIC).
MMICs are fabricated with backside metallic ground planes, to which contacts must be made from various points in the MMIC, for example at transmission line terminations. Traditionally, this has been accomplished by wire bonds. Although wire bonding techniques can be used for other devices that operate at other frequencies, they are disadvantageous at microwave frequencies in silicon carbide devices. In particular, wires tend to cause undesired inductance at the microwave frequencies at which silicon carbide devices are capable of operating. For frequencies above 10 GHz, wire bonding simply must be avoided altogether. Accordingly, such wire bonding is desirably—and sometimes necessarily—avoided in silicon carbide-based MMICs.
The use of conductive vias (i.e., via openings filled or coated with metal) to replace wire bonds is a potential solution to this problem. To date, however, opening vias in silicon carbide has been rather difficult because of its extremely robust physical characteristics, which, as noted above, are generally advantageous for most other purposes. MMICs that incorporate HEMTs and other semiconductor devices require the additional step of opening vias through the Group III nitride epitaxial layers on the silicon carbide substrate without disrupting device integrity. The invention described herein achieves the opening of conductive vias through the silicon carbide substrate and through the Group III nitride epilayers by utilizing etching techniques tailored to the chemical composition of the substrate and the epilayers.
Etching and Etchants
Etching is a process that removes material (e.g., a thin film on a substrate or the substrate itself) by chemical or physical reaction or both. There are two main categories of etching: wet and dry. In wet etching, chemical solutions are used to etch, dry etching uses a plasma. Silicon carbide does not lend itself rapidly to wet etching because of SiC's stability and high bond strength. Consequently, dry etching is most often used to etch silicon carbide.
In dry etching, a plasma discharge is created by transferring energy (typically electromagnetic radiation in the RF or microwave frequencies) into a low-pressure gas. The gas is selected so that its plasma-state etches the substrate material. Various fluorine-containing compounds (e.g., CF4, SF6, C4F8) are typically used to etch silicon carbide and different plasma reactor systems may also use gas additives such as oxygen (O2), hydrogen (H2), or argon (Ar). The plasma contains gas molecules and their dissociated fragments: electrons, ions, and neutral radicals. The neutral radicals play a part in etching by chemically reacting with the material to be removed while the positive ions traveling towards a negatively charged substrate assist the etching by physical bombardment.
Reactive ion etching (RIE) systems typically use one RF generator. The RF power is fed into one electrode (the “chuck,” on which the wafers are placed), and a discharge results between this electrode and the grounded electrode. In such systems, the capacitive nature of RF energy coupling limits the density of the plasma, which in turn leads to lower etch rates of silicon carbide. In RIE systems, plasma density and ion energy are coupled and cannot be independently controlled. When RF input power increases, plasma density and ion energy both increase. As a result, RIE systems cannot produce the type of high density and low energy plasma favorable for etching vias in silicon carbide.
In inductively coupled plasma (ICP) systems, two RF generators are used. One feeds RF power to a coil wrapped around the non-conductive discharge chamber. The second feeds power to the electrode (chuck) on which the wafers are placed. In such systems, the inductive nature of the RF energy coupling increases the efficiency of energy coupling and hence the density of the plasma. Additionally, the plasma density can be independently controlled by the coil RF power, while the ion energy can be independently controlled by the chuck RF power. Thus, ICP systems can produce the to high density and low energy plasmas that are favorable for etching vias in silicon carbide.
Etches are performed on selected areas of the wafer by masking areas of the wafer that do not need to be etched. The ratio of the etch rate of the substrate (the material to be etched) to the etch rate of the mask material is referred to as the “selectivity” of the etch. For deep etches and faithful pattern transfer, high selectivity etches are desired.
Etches generally proceed in both the vertical and horizontal directions. The vertical direction can be measured as etch depth in the unmasked areas, while the horizontal direction can be measured as undercut under the mask areas. The degree of anisotropy is expressed by how much the ratio of the horizontal etch rate to the vertical etch rate deviates from unity. When the etch rate in the vertical direction is much greater than the rate in the horizontal direction, the etch is called anisotropic. The reverse characteristic is referred to as being isotropic. Because of silicon carbide's high bond strength, it does not etch without ion bombardment in the horizontal direction. As a result, dry etches of silicon carbide are generally anisotropic.
In contrast, etches of silicon (Si) in ICP systems are generally isotropic. This results from silicon's low bond strength, because of which it readily etches in the horizontal direction. Silicon etches can be made anisotropic by using the Bosch process that alternates a deposition step for sidewall protection and an etch step.
The use of ICP (inductively coupled plasma) and ECR (electron cyclotron resonance) sources for SiC etching have resulted in higher etch rates as compared to RIE (reactive ion etch). Both ICP and ECR systems use lower operating pressure (e.g., 1 to 20 milliTorr), higher plasma density (1011 to 1012 cm−3) and lower ion energies compared to RIE systems. The combination of these parameters result in high etch rate of SiC and minimal erosion of the etch mask. RIE systems use higher pressure (10 to 300 milliTorr) lower plasma density (1010 cm−3) and higher ion energies to break SiC bonds and etch; however, the detrimental effects of high ion energies and low plasma density include mask erosion and lower etch rate.
As reported in the scientific literature by McDaniel et al., Comparison of Dry Etch Chemistries for SiC, J. Vac. Sci. Technol. A., 15(3), 885 (1997), scientists have been successful in etching SiC using an electron cyclotron resonance (ECR) plasma. Scientific studies have determined that higher ion density ECR discharges of CF4/02 or SF6/02 results in a much higher etch rate than RIE. In contrast with RIE, there have been no observed benefits to adding oxygen to either NF3 or SF6 during ECR etching.
Previous attempts at using plasma chemistries for high-density plasma etching of SiC include the use of chlorine (Cl2), bromine (Br2), or iodine (I2)-based gases. However, the use of fluorine-based gas has produced much higher etch rates. For example, Hong et al., Plasma Chemistries for High Density Plasma Etching of SiC, J. Electronic Materials, Vol. 28, No. 3, 196 (1999), discusses dry etching of 6H—SiC using a variety of plasma chemistries which include sulfur hexafluoride (SF6), chlorine (Cl2), iodine chloride (ICl), and iodine bromide (IBr) in high ion density plasma tools (i.e., ECR and ICP). These efforts have achieved etch rates of around 0.45 m/minute (4500 Å/minute) with SF6 plasmas. Alternatively, Cl2, ICl, and IBr-based chemistries in ECR and ICP sources resulted in lower rates of 0.08 μm/minute (800 Å/minute). It was found that fluorine-based plasma chemistries produced the most rapid, and hence most desirable, etch rates for SiC under high-density plasma conditions. Unfortunately, the fluorine-based chemistries displayed a poor selectivity for SiC with respect to photoresist masks.
Wang et al. reported in Inductively Coupled Plasma Etching of Bulk 6H—SiC and Thin-film SiCN in NF3 Chemistries, J. Vac. Sci. Technol. A, 16(4) (1998), the etching characteristics of 6H p+ and n+ SiC and thin-film SiC0.5N0.5 in inductively coupled plasma NF3/O2 and NF3/Ar discharges wherein etch rates of 0.35 μ/minute (3,500 Å/minute) were achieved.
In further scientific literature, Cao et al., Etching of SiC Using Inductively Coupled Plasma, J. Electrochem. Soc., Vol. 145, No. 10 (1998), discusses plasma etching in an ECR plasma using CF4 and O2 gas at flow rates of 20 standard cubic centimeters per minute (sccm) and 9 sccm, respectively, attained an etch rate in SiC of about 0.05 μm/minute (500 Å/minute). The process resulted in a 14 μm deep trench having a smooth bottom surface. Further, the low chamber pressure (i.e., 7 mTorr) minimized micromasking effects during the deep etch trenching. During the Cao et al. investigation, substrate bias was maintained at 10 V and the coil power was maintained at 700 W.
In view of the technologies discussed above, a primary objective of SiC via etching is finding a process in which SiC is etched at a reasonable rate while erosion of the etch mask is kept to a minimum. The factors affecting this objective are the choice of mask material, plasma chemistry, plasma density, and ion energy. A secondary objective when etching vias in SiC is obtaining smooth etch surfaces.
Therefore there is a need for a process in which SiC may be etched at a reasonably rapid rate while erosion of the etch mask is minimized.
There is also a need for a method for etching a via in SiC of sufficient depth and at a reasonable rate which results in a smooth surface at the bottom of the via trench.
Another need is for an etching method that efficiently etches Group III nitride epilayers without etching the contacts on a semiconductor device or any exposed silicon carbide.
A further need exists for a technique that successfully incorporates the use of appropriate vias in semi-conducting silicon carbide substrates to facilitate the manufacture of silicon carbide based MMICS and the end-use devices that can be formed with the silicon carbide-based MMICS.